Microbes Crucial to Long-Term Soil Health

Phil Robertson, Michigan State University (MSU) AgBioResearch scientist and University Distinguished Professor in the Department of Plant, Soil and Microbial Sciences has spent the past 26 years studying long-term change in the ecology of agricultural sites under the auspices of the Long-Term Ecological Research (LTER) program.

He and his colleagues at the W. K. Kellogg Biological Station (KBS) in Hickory Corners, Michigan, observe the interactions between organisms and their environment. The soil environment, one of the key components of the larger ecosystem, is of high priority in Robertson’s work.

Soil Ecosystems

One of the most important functions of the soil microbial community is to decompose and recycle organic matter left by previous crops.

Organic matter accrues in the soil in the form of aggregates, small clumps of material that build up over years into something that can be seen with the naked eye. They comprise tiny ecosystems in and of themselves. As innumerable species of bacteria and fungi consume the organic matter, nutrients are released back into the soil and taken up by the roots of crops. Given the significance of this process to crop nutrition, scientists now widely understand that soil organic matter is the basis of soil fertility.

This vital source of soil nutrients declines following conversion to conventional agriculture, however. The decline can often be attributed to increased microbial activity. As fields are tilled and soils churned, the aggregates are disturbed and exposed to the atmosphere. The sudden exposure to oxygen and the elements causes them to break apart and allows microbes instant access to the organic matter they’ve contained.

Over the course of 40 to 60 years, soil will lose between 40 and 60 percent of its original organic matter. Robertson’s team is searching for ways to reverse the deterioration scenario.

Four Agricultural Management Scenarios

As a general rule, soils are slow-changing. Though some events, such as the outbreak of a pathogen, can generate quick change, most everyday processes take years to produce a noticeable effect. This makes long-term research projects such as those at KBS essential to understanding how soil works and how it can be enhanced.

Phil Robertson

“Soil can take more than 10 years to change, which by definition is a long-term study,” Robertson explained. “It took us 10 years to document the initial changes here at KBS, and we have some studies designed to last 20 or 30 years.”

To study soil health change, Robertson’s team oversees 2.5-acre research plots with four agricultural management scenarios: conventional, no-till, reduced-input and biological. The conventional system is managed with the standard practices of the agricultural industry; no-till is a system that does not employ plows to disturb the soil. Reduced-input plots are managed with only modest amounts of synthetic fertilizers and other chemicals, and the biological system relies totally on organic sources for soil nutrients.

The team compares the results at these plots with two natural ecosystems on-site: an area formerly used for agriculture that is now returning to a natural state, and an area of old-growth forest that has never been used for anything else. In the years since the experiments began, several means of improving soil organic matter have emerged.

“By comparing these different systems, we gain tremendous insight into the factors that underlie the overall system,” Robertson explained. “The experimental plots are our most important asset in understanding the agricultural ecosystem.”

Sustainable Agriculture

For example, eliminating tilling preserves the existing organic matter while providing an environment in which more can form.

Robertson’s team witnessed firsthand how significant this practice can be during the 2012 drought, when a severe lack of rain made crops wholly reliant on water already in the soil. Though the drought proved devastating to many Michigan farmers, the higher concentrations of soil organic matter in the KBS LTER plots under no-till management allowed the fields to store an extra three inches of water, prolonging the life of their plants.

That year, the soybean yield from the no-till fields exceeded that of the conventional ones by 50 percent.

Improving soil organic matter not only has a demonstrable effect on crop yield but also helps protect the larger environment from water pollution and greenhouse gas emission. Nitrogen fixation, by which atmospheric nitrogen is converted into a form usable by plants, is an essential process fulfilled in nature largely by the bacteria that dwell in the soil.

“This is a process that has been ongoing for eons,” Robertson said. “All life depends on these bacteria, flat out.”

As soil organic matter declines, the nitrogen originally captured by these bacteria similarly declines, leaving many farmers with little recourse but to add nitrogen in the form of chemical fertilizers to fields. Applying too much fertilizer, however, or applying it at the wrong time, can cause it to leach into the groundwater and/or escape into the atmosphere, contributing to both damaging algal blooms in lakes and rivers and to climate change. Nitrous oxide that escapes into the atmosphere is approximately 300 times more effective at trapping heat than the most common greenhouse gas, carbon dioxide.

“As much as possible, we want nitrogen to originate from and stay in the cropping system,” Robertson said. “Creating an environment that encourages nitrogen conservation is beneficial not only to the plants that live there but to the planet as a whole.”